The escalating public row over the September 11 attacks reached a new pitch yesterday as the CIA director and a former White House counter-terrorism adviser clashed over whether the Bush administration had taken the al-Qaida threat seriously.

Testifying under oath at a public inquiry, Richard Clarke, formerly President Bush's top aide on counter-terrorism strategy, told relatives of September 11 victims: "Your government failed you, those entrusted with protecting you failed you and I failed you."

He accused the Bush administration of not treating the al-Qaida threat seriously and described how a top-level meeting on the issue was postponed in summer 2001, despite intelligence warnings of a major attack, because senior officials had no time in their calendars and then went on holiday.

"My view was that this administration, while it listened to me, didn't either believe me that there was an urgent problem or was unprepared to act as though there was an urgent problem," he said.

Mr Clarke's public testimony and his book on his White House experiences published this week threaten President Bush with a political crisis as he seeks to build a re-election campaign on his reputation as a determined leader in the US "war on terrorism".

The White House yesterday counter-attacked by releasing an off-the-record White House press briefing in August 2002, naming Mr Clarke as the official who gave the briefing.

In that session with reporters, Mr Clarke said the Bush administration decided immediately it came into office to "vigorously pursue the existing policy" on tracking down al-Qaida. He also declared: "There was no plan on al-Qaida that was passed from the Clinton administration to the Bush administration."

Put on the defensive, Mr Clarke said he had been acting as a loyal administration official. "I was asked to highlight the positive aspects of what the administration had done and to minimise the negative aspects of what the administration had done. And as a special assistant to the president, one is frequently asked to do that kind of thing. I've done it for several presidents," Mr Clarke said, triggering laughter in the hearing room. He denied having lied in the anonymous press briefing.

The CIA director, George Tenet, came to the president's defence with testimony to the commission which may prove to be a political lifeline for the administration.

He said that although the Bush administration may not have used the same bureaucratic methods as its predecessor, "I did not see any less attention to what we were trying to do".

However, the impact of the CIA director's testimony was partially undermined by a report delivered yesterday morning by the commission's staff. The report found that when the CIA picked up increasing numbers of signals that a major attack was imminent, some agency officials, including Mr Tenet's deputy, were impatient with the administration's response.

"Some CIA officials expressed frustration about the pace of policymaking during the stressful summer of 2001," the report found. "Although Tenet said he thought the policy machinery was working in what he called a rather orderly fashion, [the deputy CIA director, John] McLaughlin told us he felt a great tension, especially in June and July 2001, between the new administration's need to understand these issues and his sense that this was a matter of great urgency."

The report continued: "Two veteran counter-terrorism officials who were deeply involved in Bin Laden issues were so worried about an impending disaster that one of them told us that they considered resigning and going public with their concerns."